Silicon nitride based sintered products (hereinafter called silicon nitride sintered products) have high hardness and are chemically stable at high temperatures. They also have good wear resistance and good oxidation resistance. Thus, they are applicable to cutting tools.
Cutting tools must be made of a material harder than the material which is to be cut, or else the cutting tool itself would be cut. Furthermore, the tool must be able to withstand the heat and abrasive forces generated in the cutting process. Therefore, there is a need for further improvement of wear resistance and fracture resistance of silicon nitride sintered products in use as cutting tools and structural members of cutting tools.